清华大学：《计算机图形学基础》课程教学资源（授课教案）视图模型变换

Overview of Today: Some important concepts in Graphics- Color Image & Pixel (象素)- Triangle Mesh（绘制）- Lighting & ShadingTransformation (变换) and Viewing

Overview of Today • Some important concepts in Graphics – Color – Image & Pixel (象素) – Triangle Mesh – Lighting & Shading （绘制） • Transformation (变换) and Viewing

Color Perception. What is Color?- Colors are the sensations that arise from lightenergy of different wavelengths·electromagnetic waves（电磁波）withdifferentwavelengths are corresponding to different colors: Human's eye are sensitiveVisible Light Regionofthefor electromagnetic waveElectromagneticSpectrumof wavelengths between 380to 760 nm750nm600nm500nm400nm1100nmInfraredUltraViolet

Color Perception • What is Color? – Colors are the sensations that arise from light energy of different wavelengths • electromagnetic waves (电磁波) with different wavelengths are corresponding to different colors • Human’s eye are sensitive for electromagnetic wave of wavelengths between 380 to 760 nm

Spectral Distribution of LightSpectralDistribution“Light"' is a mixture of manywavelengths, each withsome intensityr- E.g., White Light means lightinclude all wavelengths withthe same intensityWavelength spectral distribution:intensity as a function ofSampleColorwavelength over the entirespectrum

Spectral Distribution of Light • ―Light‖ is a mixture of many wavelengths, each with some intensity – E.g., White Light means light include all wavelengths with the same intensity. • spectral distribution: intensity as a function of wavelength over the entire spectrum

: So, colors can be represented as suchdistribution function: However, use spectrum distribution forcolor representation is too complicated, andthe corresponds is multiple-to-one. There exits a situation as differentspectrum distribution with the samecolor""(异谱同色)

• So, colors can be represented as such distribution function • However, use spectrum distribution for color representation is too complicated, and the corresponds is multiple-to-one. • There exits a situation as ―different spectrum distribution with the same color‖ ‖(异谱同色)

RGB Color Space We first introduce color space for colorrepresentation. RBG is commonly usedcolor space in Graphics- Color is represented as RGB triple (r,g,b)- People perform almost all operations on eachchannel separately- usually, each color value is a float value rangefrom 0 to 1, or 0-255 if we use 8 bit integers

RGB Color Space • We first introduce color space for color representation. RBG is commonly used color space in Graphics – Color is represented as RGB triple (r,g,b); – People perform almost all operations on each channel separately – usually, each color value is a float value range from 0 to 1, or 0-255 if we use 8 bit integers

RGB Color Space: Color are represented as combinations ofthree primaries: red, green, blue)- C=rR+gG+ bB- The reason why we pick red, green, and blue?· essentially because of the structureof our visual system: Our visual system is sensitive to those threecolors: A demo for Color

RGB Color Space • Color are represented as combinations of three primaries: red, green, blue) – C = rR + gG + bB – The reason why we pick red, green, and blue? • essentially because of the structure of our visual system • Our visual system is sensitive to those three colors • A demo for Color

RGB Color Space: Color are represented as combinations ofthree primaries: red, green, blue)- C=rR+gG+ bB- The reason why we pick red, green, and blue?BlueGreenRed

RGB Color Space • Color are represented as combinations of three primaries: red, green, blue) – C = rR + gG + bB – The reason why we pick red, green, and blue?

RGB Color SpaceUnfortunately ,Some0.4T(2)b(a)colors cannot be可(a)0.2written ascombinations of RGBtriples, because some-0.2400700parts of the red curveWavelength (nm)is negative

RGB Color Space • Unfortunately ,Some colors cannot be written as combinations of RGB triples, because some parts of the red curve is negative

Other Color SpacesSo some other color space are also used-CMY-HSV-CIEXYZ

Other Color Spaces • So some other color space are also used – CMY – HSV – CIE XYZ

CMY? CMY :The other set of primaries besides RGBCyan(青),magenta（品红），and yellow-complements（补色）of red,green, blueRCMGAB·It's called subtractiveprimaries（减色系统）- RGB are additive primaries start with black, add upto white- CMY start with white and subtract colors from white

CMY • CMY :The other set of primaries besides RGB – Cyan(青), magenta（品红）, and yellow — complements （补色）of red, green, blue • It’s called subtractive primaries（减色系统） – RGB are additive primaries — start with black, add up to white – CMY — start with white and subtract colors from white